This invention relates to the oxidation of alkanes such as, for example, propane, to form unsaturated, carboxylic aldehydes and acids such as, for example, acrolein and acrylic acid.
Processes for producing acrylic acid by vapor phase catalytic oxidation of propylene using molecular oxygen are known and used on an industrial scale.
One of the typical processes for industrial production of acrylic acid is as follows. Propylene is converted mainly into acrolein and a small amount of acrylic acid in a first reaction step by supplying a mixture of propylene, air and steam to produce the acrolein. The acrolein product is supplied to a second reactor without separation of products for the subsequent reaction of acrolein to form acrylic acid. Additional air and steam for the second step are supplied as required.
In another typical process, the product gas containing acrylic acid obtained from the second reactor is introduced into a collecting apparatus to obtain acrylic acid as an aqueous solution and a part of remaining waste gas containing unreacted propylene from the collecting apparatus is recycled to the first reactor inlet together with the starting gas mixture of propylene, air and steam.
Various improvements to the above-mentioned processes have been proposed to produce acrylic acid efficiently by vapor phase catalytic oxidation of propylene. Many such improvements have been directed to the use of certain catalysts. Examples of catalysts used for industrial production are Moxe2x80x94Bi composite oxide catalysts for the first step, i.e., acrolein production, and Moxe2x80x94V composite oxide catalysts for the second step, i.e., acrylic acid production. There are many reasons why the characteristics of these oxidation catalysts affect the economy of the processes. Primarily, the selectivity of the catalysts for the reactions affects the quantity of propylene used, and the catalyst activity in the reactions affects the space time yield of acrylic acid.
Further enhancements directed to the use of propane as a feed source are desired because propane is more readily available and less expensive than propylene. It would be desireable if the propane could be simultaneoulsy utilized to enhance the reaction efficiency of the processes in addition to being a feed source.
By the present invention, improved continuous processes are provided for the conversion of alkanes such as, for example, propane, to unsaturated aldehydes such as, for example, acrolein, and acids such as, for example, acrylic acid.
In the processes of the present invention, an alkane having from about 2 to 8 carbon atoms per molecule, e.g., propane, is first converted to an alkene having the same number of carbon atoms as the alkane, e.g., propylene, and then alkene is converted to an unsaturated aldehyde having the same number of carbon atoms as the alkene, e.g., acrolein. The aldehyde is then converted to an unsaturated carboxylic acid having the same number of carbon atoms as the aldehyde, e.g., acrylic acid.
By operating at low propane-to-propylene conversion in accordance with the present invention, the selectivity to propylene can be made unexpectedly high, e.g., between 80 and 100 mole %. Since the presence of propane has been found to enhance the efficiency of the propylene-to-acrolein reaction, the low propane conversion is not detrimental to the process. Indeed, even though the feed to the acrolein reactor may contain propylene in low concentrations, e.g., 5 to 20 mole %, the low-conversion, high-selectivity mode of operation can be highly efficient provided unreacted propane is recycled to the propane oxidation reactor. Recycle operation is particularly feasible in accordance with the present invention because oxydehydrogenation catalysts, which are preferred for use in the present invention, are substantially unaffected by species such as carbon oxides and water which are formed in the acrolein reactor. Hence, after recovery of the acrolein, the noncondensed gases containing propane may be recycled without significant, additional purification steps.